The electrode of the invention can be employed in a wide range of electrolytic processes with no limitation, but is particularly suited to operate as an oxygen-evolving anode in electrolytic process.
Oxygen-evolving processes are well known in the field of industrial electrochemistry and include a large variety of electrometallurgical processes—such as electrowinning, electrorefining, electroplating—besides cathodic protection of cementitious structures and other non-metallurgical processes.
Oxygen is usually evolved on the surface of a catalyst-coated valve metal anode; valve metal anodes provide suitable substrates in view of their acceptable chemical resistance in most electrolytic environments, which is imparted by a very thin oxide film formed on their surface that retains a good electrical conductivity. Titanium and titanium alloys are the most common choice for the valve-metal substrate in view of their mechanical characteristics and their cost. The catalyst coating is provided in order to decrease the overpotential of the oxygen evolution reaction and usually contains platinum group metals or oxides thereof, for instance iridium oxide, optionally mixed with film-forming metal oxides such as titanium, tantalum or tin oxide.
Anodes of this kind have acceptable performances and lifetime in some industrial applications, but they are often insufficient to withstand the aggressiveness of some electrolytes especially in processes carried out at high current density, such as the case of most electroplating processes.
The failure mechanism of oxygen-evolving anodes, particularly at current density higher than 1 kA/m2, often involves a localised attack at the coating-to-substrate interface, leading to the formation of a thick insulating valve-metal oxide layer (substrate passivation) and/or to the cleavage and detachment of the catalyst coating therefrom. A way to prevent or substantially slow down such phenomena is to provide a protective barrier layer between the substrate and the catalyst coating. A suitable barrier layer should hinder the access of water and acidity to the substrate metal while retaining the required electrical conductivity. Titanium metal substrates can for instance be protected by interposing a metal oxide-based barrier layer, e.g. a barrier layer of titanium oxide and/or tantalum oxide, between the substrate and the catalyst coating. Such layer needs to be very thin (e.g. a few micrometres), otherwise the very limited electrical conductivity of titanium and tantalum oxides would make the electrode unsuitable for working in an electrochemical cell, or in any case would cause the cell voltage to increase too much with consequent increase of the electrical energy consumption needed to carry out the required electrolytic process. On the other hand, extremely thin barrier layers are liable to present fissures or other defects that can be penetrated by process electrolytes, eventually leading to harmful localised attacks.
Metal oxide-based barrier layers can be obtained in a number of different ways. For example, an aqueous solution of metal precursor salts, e.g. chlorides or nitrates, can be applied to the substrate, for instance by brushing or dipping and thermally decomposed to form the corresponding oxides: this method can be used to form mixed oxide layers of metals such as titanium, tantalum or tin, but the obtained barrier layer is generally not compact enough and presents cracks and fissures making it unsuitable for the most demanding applications. Another way to deposit a protective oxide film is by means of various deposition techniques such as plasma or flame spraying, arc-ion plating or chemical/physical vapour deposition, which are cumbersome and expensive processes that can be intrinsically difficult to scale-up as one of skill in the art readily appreciates; furthermore, these methods are characterised by a critical balance between electrical conductivity and efficacy of the barrier effect which in many cases does not lead to a fully satisfactory solution.
The simple use of a barrier layer as a protective means against corrosive attacks has always the disadvantage that inevitable local defects in the barrier structure are easily turned into sites for a preferential chemical or electrochemical attack to the underlying substrate; a destructive attack on a localised portion of the substrate can spread in many cases at the barrier-to-substrate interface and result in the electrical insulation of the substrate by virtue of a massive oxide growth and/or to an extensive cleavage of the coated components from the substrate.
The above considerations show how it is highly desirable to identify a more efficient protective barrier layer for electrodes that can be operated as oxygen-evolving anodes in electrolytic processes.